Vehicle systems may include multiple coolant loops for circulating coolant through distinct sets of engine components. The coolant flow may absorb heat from some components (thereby expediting cooling of those components) and transfer the heat to other components (thereby expediting heating of those components). For example, a high temperature coolant loop may circulate coolant through an engine to absorb waste engine heat. The coolant may also receive heat rejected from one or more of an EGR cooler, an exhaust manifold cooler, a turbocharger cooler, and a transmission oil cooler. Heat from the heated coolant may be transferred to a heater core (for heating a vehicle cabin), and/or dissipated to the atmosphere upon passage through a radiator including a fan. As another example, a low temperature coolant loop may circulate coolant through a charge air cooler. When required (such as when cabin air conditioning is requested), coolant in the low temperature loop may be additionally pumped through the condenser of an air conditioning (AC) system to absorb heat rejected at the condenser by a refrigerant of the AC system. Heat from the heated coolant may be dissipated to the atmosphere upon passage through another radiator including a fan. One example of such a vehicle coolant system is shown by Ulrey et al. in US20150047374. Another example coolant system is shown by Isermeyer et al. in US20150040874. Therein a heat exchanger enables heat exchange between a charge air cooling coolant circuit and a refrigerant circuit of the condenser.
However, the inventors herein have identified potential issues with such coolant systems. As one example, in vehicle systems where the AC system is configured to provide maximum cooling at all times, fuel economy may be lost due to the need to operate the coolant pump continuously. In addition, the pump may suffer from excess wear resulting in warranty issues. Even during steady-state conditions, the pump output may be higher than required.
Another potential issue is the position of the AC system with relation to other under-hood components. The AC condenser may be positioned at the front end of the vehicle, in front of other under-hood components, allowing a larger portion of vehicle cooling air to be directed to the condenser. However in the event of a vehicle collision, refrigerant may be lost from the condenser due to this location. This can result in additional warranty issues. On the other hand, if the AC condenser were moved further away from the front end of the vehicle to reduce such losses, the condenser may receive a smaller portion of the vehicle cooling air at the new location. Consequently there may be an increase in the need for coolant pumping through the condenser. Also, the under-hood temperature may vary from a steady-state temperature, requiring additional coolant to be pumped to carry away the under-hood heat and operate the AC system at equilibrium.
In one example, the above issues may be addressed by a method for operating a vehicle air conditioning system, comprising: adjusting, via a pump and a proportioning valve coupled to each of a charge air cooler and an air conditioner condenser, a flow of coolant through the condenser in which refrigerant different from the coolant flows, the adjusting in response to a charge air cooler coolant temperature and an actual head pressure of an air conditioner compressor. In this way, AC performance can be improved without degrading fuel economy.
As an example, each of a condenser of an AC system and a CAC may be coupled to distinct branches of a coolant circuit downstream of a proportioning valve, coolant directed into the circuit via a coolant pump. The condenser may be further coupled to a refrigerant circuit of the AC system, while the coolant circuit may be further coupled to an oil circuit of a transmission at a transmission oil cooler. The AC condenser may be positioned towards a rear end of the under-hood area. During vehicle operation, as driver demand and cabin cooling demand changes, the apportioning of coolant flow to each branch may be varied. For example, when the cooling demand of both the AC and the CAC loops is not saturated, based on the cooling demand of the AC loop, a desired coolant temperature is determined. This includes a relatively lower coolant temperature when cabin cooling demand is higher and a relatively higher coolant temperature when there is no cooling demand. Then, by referring a 2D map or model that maps a relationship between the coolant flow rate, the desired coolant temperature, and an AC head pressure, taking into account parasitic losses, a target coolant flow rate may be determined (as the point of minima of the asymptote of the 2D map). In particular, there may be a coolant flow rate above which the change in coolant temperature is not significant due to an increase in parasitic losses at the CAC. This coolant flow rate may be set as the desired coolant flow rate through the AC loop. For example, a minimum coolant flow rate may be determined when no cabin cooling is required. In addition, a corresponding reference AC head pressure may be determined. The desired coolant flow rate is then provided in a feed-forward manner via a combination of pump output adjustment and proportioning valve adjustment while taking into account constraints of other loops and other system components. Based on feedback regarding an error between the actual AC head pressure and the reference AC head pressure, it may be determined if the AC condenser is working harder than required or not, and accordingly the flow may be further adjusted. For example, if the AC is working harder than expected, coolant flow through the AC loop may be increased by increasing the opening of the valve towards the AC loop.
In this way, coolant flow may be apportioned to different components to meet their cooling demands in a fuel efficient manner. By adjusting the flow through each loop responsive to AC head pressure (instead of temperature), changes in cooling demand can be met more promptly, improving cooling response times. In addition, AC efficiency and stress can be better estimated, and compensated for. By relying on the AC head pressure for AC clutch control also, the need for additional sensors is reduced. By sharing the coolant between various components having distinct cooling demands, the need for additional radiators and fans is reduced. This allows the AC components to be placed in a rearward location of the under-hood area which reduces warranty issues without compromising cooling abilities. Overall, engine cooling performance is improved.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.